Rocky Mountain spotted fever is the deadliest tick-borne disease. If left untreated, it will kill roughly 20 percent of those who contract it, said Dr. David Walker, a professor of pathology at the University of Texas Medical Branch. Even when treated with doxycycline, it is fatal in about 4 percent of cases, he said. “You’ve got to treat them before day five if you don’t want death to start occurring,” Walker said. “It’s not an easy diagnosis to make.”